1. Field of Invention
The present invention relates to technology of setting the internal state of a current drive type pixel circuit corresponding to light emission grayscales for the current drive type pixel circuit at a high speed.
2. Description of Related Art
In recent years, an electro-optical apparatus using an organic electroluminescent (EL) element has been progressively developed. The organic EL element is a self-luminous element and does not require a backlight. Accordingly, a display apparatus using the organic EL element is expected to achieve low power consumption, a wide viewing angle, and a high contrast ratio. In this specification, the term “electro-optical apparatus” means an apparatus that converts electrical signals into light. The electro-optical apparatus normally converts electrical signals representing an image into light representing the image and is particularly suitable to implementation of a display apparatus.
FIG. 13 is a block diagram of a conventional display apparatus using an organic EL element. The conventional display apparatus includes a display matrix section (hereinafter, referred to as a “display region”) 120, a scanning line driver 130, and a data line driver 140. The display matrix section 120 includes a plurality of pixel circuits 110 arranged in a matrix. Each pixel circuit 110 includes an organic EL element 220. Each of the pixel circuits 110 arranged in a matrix is connected to one of a plurality of data lines Xm (where m=1, 2, . . . , and M) extending in a column direction and is connected to one of a plurality of scanning lines Yn (where n=1, 2, . . . , and N) extending in a row direction.
FIG. 14 is a circuit diagram illustrating an example of the pixel circuit 110. The pixel circuit 110 is located at an intersection of an m-th data line Xm and an n-th scanning line Yn. The scanning line Yn includes two sub-scanning lines V1 and V2. The pixel circuit 110 is a current drive type circuit that controls a light emission grayscale of the organic EL element 220 corresponding to a current flowing in the data line Xm. In detail, the pixel circuit 110 further includes four transistors 211 to 214 and a storage capacitor 230 in addition to the organic EL element 220. The storage capacitor 230 stores charges corresponding to data signals received via the data line Xm to control the light emission of the organic EL element 220 using the stored charges. In other words, the storage capacitor 230 stores a voltage corresponding to the current flowing in the data line Xm. The first to third transistors 211 to 213 are n-channel field effect transistor (FET) and the fourth transistor 214 is a p-channel FET. The organic EL element 220 is a current drive type light emission element like a photodiode and is thus marked with a symbol of a diode in the drawings.
The source of the first transistor 211 is connected the drain of the second transistor 212, the drain of the third transistor 213, and the drain of the fourth transistor 214. The drain of the first transistor 211 is connected to the gate of the fourth transistor 214. The storage capacitor 230 is connected between a source and the gate of the fourth transistor 214. The source of the fourth transistor 214 is connected to a power supply voltage Vdd.
The source of the second transistor 212 is connected to the data line driver 140 via the data line Xm. The organic EL element 220 is connected between the source of the third transistor 213 and a ground voltage. The gate of the first transistor 211 and the gate of the second transistor 212 are commonly connected to the first sub-scanning line V1. The gate of the third transistor 213 is connected to the second sub-scanning line V2.
The first and second transistors 211 and 212 are switching transistors used to accumulate charges in the storage capacitor 230. The third transistor 213 is a switching transistor that is in an ON state during the light emission of the organic EL element 220. The fourth transistor 214 is a driving transistor that controls a value of current flowing in the organic EL element 220. The current value in the fourth transistor 214 is controlled by the amount of charges stored (i.e., accumulated) in the storage capacitor 230.
FIG. 15 is a timing chart illustrating the normal operation of the pixel circuit 110. In FIG. 15, a voltage in the first sub-scanning line V1 (hereinafter, referred to as a first gate signal V1), a voltage in the second sub-scanning line V2 (hereinafter, referred to as a second gate signal V2), a current in the data line Xm (hereinafter, referred to as data signals Iout), and a current IEL in the organic EL element 220 are represented.
A driving period Tc is divided into a programming period Tpr and a light emission period Tel. The driving period Tc is a period of time taken to update a light emission grayscale of each of the organic EL elements 220 within the display matrix section 120 one time. The driving period Tc is referred to as a frame period. A grayscale update is performed in a group of pixel circuits in a single row at one time and is sequentially performed in N groups of pixel circuits in the N rows during the driving period Tc. For example, when the grayscale update is performed on all of the pixel circuits 110 at 30 Hz, the driving period Tc is about 33 ms.
The programming period Tpr is a period of time while the light emission grayscales of each organic EL element 220 is set in a corresponding pixel circuit 110. Here, programming indicates the operation of setting the light emission grayscale in the pixel circuit 110. For example, when the driving period Tc is about 33 ms and the total number N of the scanning lines Yn is 480, the programming period Tpr is less than about 69 μs.
During the programming period Tpr, the second gate signal V2 is set to a “low” level and the third transistor 213 remains turned off. Next, a current Im corresponding to the light emission grayscale flows in the data line Xm, the first gate signal V1 is set to a “high” level, and the first and second transistors 211 and 212 are turned on. Here, the data line driver 140 functions as a constant current source that provides the current Im according to the light emission grayscale.
Charges corresponding to the current Im flowing in the fourth transistor 214 (i.e., the driving transistor) are stored in the storage capacitor 230. As a result, a voltage stored in the storage capacitor 230 is applied between the source and the gate of the fourth transistor 214. Hereinafter, the current Im of data signals used in the programming is referred to as a “programming current Im”. After the programming is finished, the scanning line driver 130 sets the first gate signal V1 to the “low” level and turns off the first and second transistors 211 and 212. The data line driver 140 stops outputting the data signals Iout.
During the light emission period Tel, while the first gate signal V1 remains at the “low” level, the first and second transistors 211 and 212 remain turned off, the second gate signal V2 is set to the “high” level and the third transistor 213 is turned on. Since the voltage corresponding to the programming current Im has been stored in the storage capacitor 230, almost the same current as the programming current Im flows in the fourth transistor 214. Therefore, almost the same current as the programming current Im flows in the organic EL element 220. The organic EL element 220 emits light with a grayscale corresponding to the current value Im.
In the display apparatus illustrated in FIG. 13, the light emission of the organic EL element 220 included in each pixel circuit 110 is controlled according to the above-described sequence of operation. However, when a large display panel is manufactured using the above-described structure, the capacitance (Cd) of each data line increases and a large amount of time is required to drive the data lines. To solve these problems, “Patent Document 1” discloses technology for accelerating charge or discharge by writing the power supply voltage Vdd in the data line Xm connected to the pixel circuit 110 before programming a current corresponding to the light emission grayscale in the pixel circuit 110, that is, before setting an internal sate of the pixel circuit 110. Hereinafter, the operation of programming a predetermined voltage in a data line connected to a current drive type pixel circuit before the internal state of the pixel circuit is set corresponding to the light emission grayscale of the pixel circuit, thereby accelerating the charge or discharge, which is referred to as “precharging”. A voltage written in the data line by the precharging is referred to as a “precharge voltage”.
[Patent Document 1] Pamphlet of PCT Publication WO 01/006484